Background: Longitudinal studies estimating the association between disability and mortality in the elderly population have typically assumed disability constant through the follow-up study period. Current knowledge indicates that such assumptions may not be appropriate. Our purpose was to examine this association (disability and mortality) taking into account the transitions in the disablement process.
Methods: 1,294 participants (aged 65 and over) in the Health Interview Survey of Barcelona were followed up for a median of 8 years. Nine basic activities of daily living (ADLs) were measured at baseline and at the end of follow-up. Individuals were defined as "dependent" if they reported not being able to perform one or more of the activities without assistance. Survival analysis with delayed entry, age as the time variable, and disability as a time-dependent variable was performed.
Results: The rates of disability had increased by the end of the follow-up (from 42.0 to 60.0% among women and from 30.0 to 48.0%, among men); 7.5% of disabled women at baseline and 28.5% of men recovered from disability. The adjusted relative risk of dying for those with basic ADLs dependency varied with age: at 80 years of age it was 3.5 for women and 1.8 for men, while at 90 years it was 1.9 and 1.2, respectively.
Conclusions: Disability increases monotonically over time while the risk of mortality associated with disability varies with gender and age. Elderly disabled women should be considered a target group for intervention because they show higher rates of disability and are less likely to recover from disability. Our results illustrate the need to consider disability status as a time-dependent variable, to avoid an underestimation of its association with mortality.